A combination of hot rolling and equal channel angular pressing (ECAP) was explored to generate globular microstructures in the Mg-3%Zn alloy after re-heating to the semisolid state. It was found that the single-step deformation of as-cast alloy via hot rolling at 350°C with a thickness reduction of 50% refined the alloy microstructure by creating deformation bands of the Mg (α) phase with a size of the order of tenths of micrometers. After re-heating to 630 °C, the microstructure transformed into spheroidal morphologies with an average globule size of 82 μm. An additional deformation of the hot-rolled alloy by the ECAP method at 250 °C further refined the alloy microstructure to sub-micrometer grains of lath and equiaxed shapes. After re-heating of this microstructure to 630 °C the average globule size reached 62 μm, which is roughly 25% smaller than that achieved for the hot-rolled precursor. The role of strain-induced melt activation (SIMA) techniques in generation of globular morphologies in Mg-based alloys after partial re-melting is discussed.
Abstract The electron beam additive manufacturing (EBAM) method was applied in order to fabricate rectangular-shaped NiTi component. The process was performed using an electron beam welding system using wire feeder inside the vacuum chamber. NiTi wire containing 50.97 at.% Ni and showing martensitic transformation near room temperature was used. It allowed to obtain a good quality material consisting of columnar grains elongated into the built direction growing directly from the NiTi substrate, which is related to the epitaxial grain growth mechanism. As manufactured material showed martensitic and reverse transformations diffused over the temperature range from −10 to 44 °C, the applied aging at 500° C moved the transformation to higher temperatures and transformation peaks became sharper. The highest recoverable strain of about 3.5% was obtained in the as-deposited sample deformed along the deposition direction. In the case of deformation of the alloy aged at 500 °C for 2h, the formation of martensite occurs at significantly lower stress; however, at about 2.5% the stress begins to increase gradually and only a small shape recovery was observed due to a higher martensitic transformation temperature. In situ SEM tensile deformation in the direction perpendicular to deposition direction showed that the martensite began to appear at the surface of the sample and at the grain boundaries due to heterogeneous nucleation. In situ studies allowed to determine the following crystallographic relationships between B2 and B19’ martensite: (100) B2 ||(100) B19’ and (100) B2 || (011) B19’ ; (011)B2|| (001) B19’ and $${(011)}_{\mathrm{B}2}||{\left(11\bar{1 }\right)}_{\mathrm{B}1{9}^{\mathrm{^{\prime}}}}$$ (011)B2||111¯B19′ . Samples aged at 500 °C exhibited fully austenitic microstructure; however, with increasing degree of deformation, the formation of martensite was observed. The majority of needles were tilted about 45° with respect to the tensile direction, and the presence of type I (11 $$\bar{1 }$$ 1¯ ) invariant twin boundaries was observed at higher degrees of deformation.
Influence of HIgH-TemperaTure oxIdIzIng condITIons on alcocrcunI HIgH enTropy alloys wITH and wITHouT sIlIcon addITIon initial investigations on oxidation behaviour and phase transformations of equimolar alCoCrCuni high entropy alloy with and without 1 at.%silicon addition during 24-hr exposure to air atmosphere at 1273 K was carried out in this work.after determining the oxidation kinetics of the samples by means of thermogravimetric analysis, the morphology, chemical and phase compositions of the oxidized alloys were determined by means of scanning electron microscopy, energy dispersive X-ray spectroscopy and X-ray diffraction analysis.additional cross-section studies were performed using transmission electron microscopy combined with energy dispersive X-ray spectroscopy and selected area electron diffraction.From all these investigations, it can be concluded that minor silicon addition improves the oxidation kinetics and hinders the formation of an additional FCC structure near the surface of the material.
Summary Amorphous powder of composition corresponding to Ni60Ti20Zr20 (in at%) was obtained by ball milling in a high‐energy mills starting from pure elements. Formation of the amorphous structure was observed already after 20 h of milling, although complete amorphization occurred after 40 h. The microhardness of powders increased from about 30 HV for pure elements to above 400 HV (1290 MPa) after 40 h of milling. Transmission electron microscopy (TEM) allowed to identify nanocrystalline inclusions of intermetallic phases of size 2–10 nm. Uniaxial hot pressing was performed in vacuum at temperature below the crystallization T x it is 510°C and pressure of 600 MPa, Mixed amorphous powders and nanocrystalline silver powders were used to form a composite, in which microhardness was near 970 MPa HV and 400 HV for the amorphous phase and nanocrystalline silver, respectively. The compression strength of the composite containing 20 wt% of nanocrystalline Ag powder was equal to 600 MPa and plastic strain was 2%. Microstructure studies showed low porosity of composites of less than 1%, uniform distribution of the silver phase and a transition zone between both components, about 150 nm thick, where diffusion of nickel, niobium and zirconium into silver was observed. High‐resolution TEM allowed identifying the structure of nanocrystalline inclusions in the amorphous matrix after hot pressing as either Ni 3 Zr or Ni 17 Nb 3 . The identification was performed basing on measurements of angles and interatomic distances using inverse Fourier transformed images with enhanced contrast using Digital Micrograph computer program.
Comparison of miCrostruCture, ageing effeCt and shape memory properties of additively manufaCtured niti alloy using lens and eBam methods samples prepared using various additive manufacturing methods were compared in terms of structure, texture, transformation temperature and superelastic properties.samples manufactured using laser engineered net shaping (leNs) method showed texture several degrees deviated from the <001> build direction, however with composition near to the initial powder composition, enabling superelastic effect.The electron beam additive manufacturing (eBaM) samples showed martensitic structure at room temperature due to a shift of transformation temperatures to the higher range.This shift occurs due to a lower Ni content resulting from different processing conditions.However, eBaM method produced sharper <001> texture in the build direction and made it possible to obtain a good superelastic effect above room temperature.intermetallic particles of size 0.5-2 mm were identified as Ti 2 Ni phase using eDs and electron diffraction analyses.This phase was often formed at the grain boundaries.Contrary to the leNs method, the eBaM prepared samples showed Ni-rich primary particles resulted from different processing conditions that reduce the Ni content in the solid solution thus increase the martensitic transformation temperature.ageing at 500°C allowed for shifting the martensitic transformation temperatures to the higher range in both, leNs and eBaM, samples.it resulted from the formation of Ni rich coherent precipitates.in samples prepared by both methods and aged at 500°C, the presence of martensite B19' twins was observed mainly on {011} B19' planes.
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